1. Field of the Invention
The present invention relates to a main electrode for use in plasma arc working torch which is capable of welding or cutting works.
2. Description of the Prior Art
A plasma arc working torch known in the prior art is generally in a structure shown in FIG. 7 wherein reference numeral 1 designates a plasma electrode which is cooled by a cooling agent. The electrode 1 is composed of an base electrode 2 in a pipe form and an insert of refractory metal 3 inserted in a hollow portion at the end of the base electrode 2. The base electrode 2 can be made of copper metal or copper alloy while the refractory metal can be made of hafnium metal or zirconium metal. Reference numeral 4 designates an electrode supporting member for supporting the electrode 1, which is made of electrically conductive material. Reference numeral 5 designates an insulating sleeve formed at the outside of the electrode supporting member 4. Reference numeral 6 designates a tip supporting member which is formed at the outside of the insulating sleeve 5 and is made of electrically conductive material. A torch body 7 is constructed from the electrode supporting member 4, the insulating sleeve 5 and the tip supporting member 6.
Reference numeral 8 designates a tip electrode in a hollow form supported at the end of the tip supporting member 6. The tip electrode 8 has a plasma jet hole 801 formed at the center of the end thereof. Reference numeral 9 designates an insulating cap and reference numeral 10 designates a guide pipe for cooling water. Cooling water supplied from a supplying hose 11 cools directly the main electrode 1 and flows into the path shown by an arrow and finally goes out from the torch trough a drain hose 12.
In the torch mentioned above, an electric power is supplied between the main electrode 1 and the work while a plasma forming gas such as air, oxygen gas or nitrogen gas is spouted from the plasma jet hole 801 at the tip electrode 8 to generate a plasma jet. The working of the work can be carried out with this plasma jet.
In the operation of the torch shown in FIG. 7, high voltage of a high frequency generated by a high frequency generator 14 is applied, through a capacitor 15, between the main electrode 1 and the tip electrode 8 to generate a so-called pilot arc. This pilot arc is spouted from the plasma jet hole 801 of the tip electrode 8 by the action of a flow of the plasma forming gas. When the torch (T) is brought near the work 13 with keeping the pilot arc, a working arc is generated between the main electrode 1 and the work 13. When the working arc has been generated once, the pilot arc at the tip electrode disappears because there is a resistor 16 on the way of the electric path for generating the pilot arc. It should be noted that the high frequency generator 14 stops its operation with the generation of the pilot arc.
The plasma arc working torch in the structure mentioned above has the following disadvantage: The main electrode 1 is cooled always but is heated up to a high temperature during the working time. U.S. Pat. No. 3,597,649 discloses the main electrode 1 composed of the base electrode 2 and an insert of refractory metal 3 such as hafnium inserted into the hollow of the end of the base electrode 2. However, even with this main electrode 1, the operation life is still short due to the high temperature of the main electrode.
On the other hand, the U.S. Pat. No. 3,198,932 discloses the main electrode 1 in which a high-heat insert 3 of zirconium refractory metal is plated with zinc film by immersing into a molten zinc chloride and further plated with silver film by immersing into a molten silver metal. The high-heat insert 3 of zirconium refractory metal having a zinc film and a silver film plated sequentially thereon is soldered to the hollow of the end of the base electrode 2 by using silver soldering material. In this case, a zinc oxide film is formed on the surface of the plated zinc film and prevents the heat transmission from the zinc film to the silver film. As a result, the heat generated at the high-heat insert 3 of zirconium refractory metal is not conveyed rapidly to the base electrode 2. This does not result in an improvement in the operation life of the main electrode 1 as high as expected. Further, the zinc film obtained by immersing the high-heat insert 3 of zirconium refractory metal into the molten zinc chloride separates easily from the insert of zirconium refractory metal 3. Therefore, the plated insert of zirconium refractory metal 3 is undersirably apt to have the plated films separated easily therefrom when subjected to the external force during a working time period until the completion of the silver soldering work to the hollow of the end of the base electrode 2. Further the insert 3 of zirconium refractory metal is heated up to a high temperature during the work of the plasma arc working torch. As a result, the silver soldering material for soldering the insert 3 of zirconium refractory metal to the hollow of the end of the base electrode 2 melts and forces the insert 3 to separate from the base electrode 2.
Further, U.S. Pat. No. 3,944,778 describes an improved main electrode for use in a plasma arc working torch in a structure as described below. A cooling holder 1 is made of an electrically conductive metal having a high thermal conduction such as copper. There is provided a room 7 between the cooling holder 1 and a relating thin insert 2 of a refractory metal. The room 7 is fulfilled with a material having a lower thermal conduction than that of the cooling holder 1. Since the thermal conduction of the material fulfilling the room 7 is lower than that of the cooling holder 1, the heat transmission from the periphery of the thin insert of a refractory metal 2 is higher than that from the center of the thin insert of a refractory metal 2. That is, the purpose of this structure is to localize the arc generating point to the effective center of the thin insert 2 of a refractory metal by over-heating forcedly the center of the thin insert 2. In other words, the temperature distribution at the working surface of the thin insert 2 is controlled by over-heating forcedly only the center of the thin insert. It is necessary for the achievement of this effect to make the thin insert 2 thinner, that is, to make the height of the thin insert 2 lower than the diameter thereof. Such a thin insert 2 of a refractory metal having a height smaller than the diameter undesirablly results in a short operation life of the plasma arc working torch.